I wanted to make sure if I had to change this in my car or not. I've heard if you change your wheel size/tire size, you've got to re-calibrate the PCM.

Since I did my tune, my milage went in the toilet, pretty much from 19 mpg, to 14.6 mpg. I went to a custom 91 octane tune using a sct live wire tuner.

In any case, would going from 17" x 7" wheels and stock tires to 18x10 and 275x40 tires in the rear and 265x35x35 tires in the front require a revs per mile change in the PCM?

I don't think the tune is having any issues, I'm not getting any valve knock, back firing, or improper detonation. I just want to be sure since I know this can make you look like you're altering the milage.

 

Try this & yes you will: http://www.1010tires.com/TireSizeCal...&reset=yes

Rick

 

Awesome, thanks for the link.

I'm assuming you would set the revs per mile for the numbers generated on the rear tires right.

 

yup.

Rick

 

That website is close, but every tire is a bit different. I prefer to use the manufacturers website. That site says 780, but the manufacturer says 783. So you would want to select 785 in your tuner, because that's closer. That is, if you are running the Michelin Pilot Sport PS2's like it says in your signature.

I dunno why everyone keeps going with that tire size. It's half an inch smaller than stock! And your fronts are almost 2 inches smaller than stock. Yikes! You wouldn't have had to go with such a big drop with larger tires and would have gotten more clearance for the underside of the car. Just so you know, when it's time for new tires.

 

Nearly 2 inch suspension drop, it matches up.

 

Wrap a tape measurer around the mounted tire to get the distance around it.
Divide 63360 (inches per mile) by the inches around to get actual REVS PER MILE
for any given tire size.

http://www.1010tires.com/TireSizeCal...?action=submit
http://www.miata.net/garage/tirecalcold.html